Today passive optical network (PON) systems exist where an optical line terminal (OLT), usually located at a providers premises, is connected by optical fiber to a number of optical network units (ONUs) usually located at a residential premises or business premises. To reduce costs these networks are often passive, hence called optical PON networks, which usually means that the routing devices between the OLT and the ONUs do not require any power or light sources. This reduces costs since optical sources in the form of transmitters are required only in each optical network unit (ONU) and in the optical line terminal at the providers premises.
In a simple arrangement, one wavelength is used for downstream signals, i.e. signals from the OLT to the ONUs, and time division multiplexing (TDM) is used to allocate a portion of the downstream signal to each ONU. A different wavelength may be used for upstream signals, i.e. signals from ONUs to the OLT. A simple passive splitter can then be used to send the correct portion of signal to each ONU.
In a refinement it is also known to use wavelength division multiplexing (WDM) where optical signals at more than one wavelength are sent out by the OLT, with each ONU being allocated one wavelength (although they could be allocated more than one). Each signal can then be modulated with information to be sent to an ONU. To route the correct wavelength signal to the correct ONU, a passive distribution node is provided which taps into the optical fiber at a location between the OLT and the ONUs. The function of the node, which often is referred to as a wavelength multiplexer (WMUX), is to combine upstream channels from different OLTs and to separate downstream channels to different OLTs. In this context “upstream” refers to data transmission from an ONU to an OLT while “downstream” refers to data transmission from an OLT to an ONU. As indicated, the upstream channels from different ONUs typically have different upstream wavelengths and are combined in the WMUX and is thereafter received by the OLT as an aggregated channel. The OLT can then separate the channels from each ONU by virtue of the different upstream wavelengths. In the downstream an aggregated channel from the OLT carries a downstream channel for each ONU, and the WMUX splits the aggregated channel into downstream channels that are received by a respective ONU via a respective optical communication line.
The WMUX is often of the arrayed waveguide grating (AWG) type, which is a known device for separating light (channels) of different wavelengths and to send them to the correct ONUs.
A wavelength division multiplexing passive optical network (WDM-PON) has in comparison with a time division multiplexing passive optical network (TDM-PON) several advantages, such as increased network capacity, dedicated bandwidth to each end-user (i.e. to each ONU), communication privacy and lower insertion loss of an WMUX compared to e.g. a power splitter in a TDM-PON, which in turn enables long reach. A WDM-PON has however a disadvantage in that each ONU much transmit signals (i.e. use a communication channel) to the OLT at a specific wavelength. Since it is usually not practical to implement a large number of different ONU-types, wavelength adaptive ONU-transmitters must often be used. This is typically referred to as “colorless” ONUs.
Tunable lasers as transmitters in an ONU are widely considered as the best long term solution for enabling upstream transmission at a correct wavelength. However, tuning of the transmitters must also be performed, both at a first the initialization of the transmitter as well as during operation of the transmitter.
Today solutions exist where a transmitter of an ONU is tuned for using a correct wavelength for an upstream optical signal.
KR2007059895, for example, discloses a WDM-PON and a wavelength initialization method that provide automatically array tunable light sources according to allocated unique wavelengths. This is achieved by installing WDM-PON master controllers and a WDM-PON slave controller in an OLT and an ONU respectively. The system and method are using a table configuration algorithm and an optimal value determination algorithm for performing an automatic wavelength initialization function.
US2007/0133986 discloses another technique for setting an upstream wavelength used for transmission from an optical network terminal acting as an ONU. Here, an OLT monitors an upstream optical wavelength and determines whether the wavelength is proper, to decide adjustment/maintenance and transmits a proper message as a result of the decision to the optical line terminal downwards.
The techniques described above are generally capable of adjusting an upstream wavelength used by an ONU for transmission to an OLT, but suffer from being relatively complex and/or an inability to satisfactory handle setting of upstream wavelengths for a larger number of ONUs. This is mainly due to the fact that the OLTs actively determines which upstream wavelength the ONU shall use, i.e. the OLT controls the ONU such that the ONU will set the correct upstream wavelength, which requires a rather complex structure of the OLT, in particular if a large number of ONUs are connected to the OLT.